fix(ui,player): scroll restore, immersive fullscreen, watched toggle, handoff timeline, PiP

Batch of reported bugs and enhancements.

UI
- Pages no longer inherit the previous page's scroll position (DR-156, UR-072).
  The shell keeps its scrollers alive across navigation by design, so the
  element never remounts and its scrollTop survived the route change; SvelteKit
  restores window scroll, which this app never uses. ScrollMemory records the
  offset per route and per container: forward moves reset to the top, Back
  restores where the route was left.
- Season header stacks on narrow screens, and the title span gets min-w-0 so it
  actually truncates instead of overflowing under the action buttons.
- Favourites gets a labelled tile at the head of the library grid rather than
  only an unlabelled heart icon in the header.

Playback
- Full-screen video on Android hides the system bars (DR-157, UR-066).
  requestFullscreen() cannot touch the Activity window from inside a WebView, so
  the control did nothing visible while the bars stayed painted over the video.
  ImmersiveModeBridge hides them, restored on exit, Escape and teardown.
- Background-audio handoff stops leaking its relative timeline (DR-159).
  background_audio_base was a display-only correction applied in two places
  while progress reports to Jellyfin, the frontend and media3's own seeks all
  worked in the relative timeline treating it as absolute — each crossing losing
  exactly `base` seconds. The conversion now happens once, in the position tick,
  and inbound seeks resolve through seek_absolute, which re-opens the stream at
  the requested position because the handoff transcode cannot seek.
- Picture-in-picture works on the path that actually plays video (DR-160).
  canEnterPip demanded a native ExoPlayer surface, but that path is behind a
  flag defaulting to off, so PiP could never engage. It now accepts the WebView
  <video> too, keeping the WebView visible and routing play/pause to the element.
- Native video is now the default so PiP has a real surface (DR-161). The
  scrub-regression tests pinned the flag-off path implicitly; they now mock it
  off explicitly. The native scrub/seek path is not covered by the suite and
  needs device verification.

Watched state
- Watched toggle on the episode row, season header, series and movie hero, and
  the Episode Focus View (DR-158, UR-073). Both backend halves already existed
  with no caller. storage_set_watched covers a container's episodes so the
  toggle is honest offline, and QueuedOp::MarkUnplayed gives the sync queue the
  missing direction.

Release
- Fix the Android versionCode floor (set-version.sh). v0.5.2 shipped code 5002
  under an earlier minor*1000 scheme, but the current minor*100 formula yields
  1502 for that version and 1503 for 0.5.3 — so every 0.5.x release built from
  it was an un-installable downgrade for anyone already on v0.5.2. Widened to
  10000 + major*1000000 + minor*1000 + patch (0.5.3 -> 15003).
- Bump to 0.5.3.

src/routes/+layout.svelte

@@ -31,6 +31,7 @@
     shellReservesBottomInset,
   } from "$lib/utils/layoutShell";
   import { registerNavigationTracking } from "$lib/utils/navigation";
+  import { useScrollRestore } from "$lib/utils/scrollContainer";
   import { startNetworkReporting } from "$lib/services/networkType";
   import { initSafeArea } from "$lib/utils/safeArea";
 
@@ -52,6 +53,12 @@
   // context, not the async onMount callback below.
   registerNavigationTracking();
 
+  // The shell's scroller outlives every route rendered into it, so without this
+  // a new page inherits the previous page's offset. Must be registered here at
+  // init, alongside the tracker above, for the same reason. (DR-156)
+  let shellScroller = $state<HTMLElement>();
+  useScrollRestore(() => shellScroller, "shell");
+
   // Layout-shell visibility rules live in one pure, unit-tested module
   // ($lib/utils/layoutShell) so they can't drift per route/platform.
   //
@@ -313,6 +320,7 @@
            sibling, so the list is physically bounded above it and can never
            render behind it. No measurement, no reserved padding. -->
       <div
+        bind:this={shellScroller}
         class="flex-1 overflow-y-auto min-h-0"
         style="overscroll-behavior: contain"
       >